The biotechnology industry has created a number of successful biological drugs for treating chronic medical conditions. Such drugs include including alpha epoetin (Procrit®, Epogen®) for treatment of chronic anemia associated with cancer chemotherapy, granulocyte colony stimulating factor (Neupogen®) for treatment of neutropenia associated with cancer chemotherapy; alpha interferon (Intron® A. Roferon® and Infergen®) for treatment of chronic hepatitis; and, beta Interferon (Avonex®) for treatment of relapsing multiple sclerosis.
Although these products provide important benefits to patients and generate sales measured in billions of dollars, these and most other biological drugs have two major limitations: (i) due to their large molecular weight (e.g., >10,000 Daltons) and fragility, such drugs cannot be delivered to the patient by the oral route, and thus, injection is the only method of administration; and (ii) their typically short half-life results in the drugs being quickly cleared from the body, and therefore, they must be administered to the patient frequently (e.g., daily or three times per week).
In the hospital setting, intravenous administration is usually a safe and reliable method for administering biological drugs. In medical situations where hospitalization or physician visits are not necessary, patients often self-administer biological drugs by subcutaneous or intramuscular injection several times a week over the course of therapy. However, this type of therapy is generally associated with pain at the site of injection, injection site reactions, infections, and lack of compliance with dosing schedule.
Sustained release implants or drug depots provide a potential solution to the medical need for delivering biological drugs for chronic conditions. Sustained release implants have the potential to eliminate compliance as a concern because they provide the physician with the assurance that the drug is being delivered and the patient with the freedom to go about their normal daily activities. Currently, two basic technologies have been developed to address the medical need for sustained release of chronically administered injectable drugs: injectable erodable polymer depots designed to act for several weeks, and implantable devices capable of delivering potent drugs for up to one year. While effective in certain cases, these prior art devices are subject to important limitations.
Sustained release depot formulations that employ polymer depots typically exhibit an initial “burst effect” resulting in the release of up to 90% of the encapsulated drug in the first few days after implantation. Following injection of the device, plasma levels quickly peak and then decline to near constant levels. This characteristic of depots makes them unsuitable for sustained release of a drug over time, where a more constant rate of delivery is desired.
Other prior art implantable devices utilize a semi-permeable membrane to cause osmotic tablets to slowly swell as they absorb water. The swelling tablets push a piston that forces drug from a reservoir out of a small opening. Such devices are capable of sustained release over longer periods of time; however, the number of drugs that are compatible with such devices is limited due to the construction of the device. Thus, only highly potent drugs such as certain hormones can be successfully used with these prior art devices.
Thus, although current technologies provide important advantages over traditional daily injections, a need currently exists for implantable systems that are more flexible with respect to the types, size, stability and solubility properties of drugs themselves, and with respect to the drug delivery patterns achievable using such a device, i.e., avoidance of “burst” effects and achieving a more constant rate of drug delivery.